The invention relates to a method for operating a particle filter provided for filtering out particles contained in the exhaust of motor vehicle combustion engines.
It is customary to use particle filters for filtering out particles in the exhaust of motor vehicle combustion engines, in particular of diesel engines. Wall-flow particle filters of honeycomb structural form or sintered metal filters are frequently used. Such filters generally permit the removal of 90% or more of the particle mass or number of particles contained in the exhaust. The particles, which are present in the exhaust substantially as soot particles, are filtered out on the basis of a surface and/or depth filtration effect and over time accumulate in the particle filter. This undesirably increases the flow resistance of the particle filter, which is why the accumulated soot particles have to be removed from time to time by a regeneration operation. This is usually done by an enforced soot burn-off at an elevated temperature of typically more than 600° C. The necessity of such a regeneration operation by thermal soot burn-off is mostly carried out by monitoring the pressure drop across the particle filter or using model calculations for the particle loading of the particle filter. In particular when using a calculation model for determining the particle loading, it is disadvantageous if the behavior of the particle filter with regard to the efficiency of the filtering out of particles, i.e., the separation efficiency, changes over the period of use of the particle filter. Also, for reasons of operating reliability, a separation efficiency that changes is disadvantageous, in particular if at times it is lower than is necessary or desired.
Exemplary embodiments of the present invention are directed to a method for operating a particle filter for filtering out particles contained in the exhaust of motor vehicle combustion engines in which the aforementioned disadvantages are avoided.
In the method according to the invention for operating a particle filter provided for filtering out particles contained in the exhaust of motor vehicle combustion engines, in a new condition of the particle filter a conditioning step is performed such that the separation efficiency of the particle filter for particles is increased relative to the value existing in the new condition. As the inventors have established, in many cases the separation efficiency of a new, serviceable particle filter initially is more or less considerably less than desirable and necessary for filtering out or reducing particles at a level required in normal use. In addition, the separation efficiency proves not to be constant, but increases gradually for a new particle filter at the beginning of its period of use. This is also disadvantageous insofar as, especially at the start of the period of use, particle reduction values may fall short of those prescribed or sought. The drift over time also makes estimating the particle loading more difficult. Models intended for estimating or calculating the particle loading fail or provide inaccurate results. This makes it difficult, in particular at the beginning of the period of use, to reliably determine regeneration timings. Overall, the operation of a particle filter, owing to the phenomenon established by the inventors, especially at the beginning of its use, disadvantageously proves unreliable and difficult to calculate. These problems are not, or only insignificantly, alleviated by the gradually occurring stabilization with regard to the separation efficiency that is likewise observed. As a result of the conditioning step provided according to the invention, the disadvantageous phenomenon of a separation efficiency, which is reduced in the new condition and increases over the course of time is at least largely eliminated.
Provision is made for the conditioning step for a new, ready for use particle filter to be carried out with the operating method usually provided before the intended use thereof. Before performing the conditioning step, the manufacturing process for the particle filter is terminated and the particle filter is ready for its use in the vehicle for filtering out particles. Nevertheless, the conditioning step may succeed the manufacturing process before the particle filter starts operating. Provision is made, after the conclusion of the conditioning step, for the separation efficiency to be increased from a lower starting value to more than 70%, preferably to more than 90%, and particularly preferably to more than 97%, relative to the particle mass or the number of particles of the filtering out of particles upon the normal intended use of the particle filter. In such case, provision is made for the conditioning step to be carried out such that the separation efficiency of the particle filter after conclusion of the conditioning step upon the subsequent intended use remains substantially constant above a threshold value which is increased relative to the starting value.
The invention is advantageously applicable to open pored or closed pored filters with predominantly surface or depth filtration action. In particular, the invention is applicable to wall-flow particle filters of honeycomb structural form. With these particle filters, a flow through the porous channel walls which is forcibly brought about by end-face closure of elongate inflow channels results in a filtration effect. In a particularly advantageous manner, the invention is applicable to what are called wall-flow filter types of this type on the basis of silicon carbide, cordierite and/or aluminum titanate. However, the invention is also applicable to other filter types, such as for example foam type filters on a ceramic or metal basis or to sintered metal filters.
The conditioning step for increasing and stabilizing the separation efficiency is carried out according to the invention when the particle filters are in the new condition. A “new condition” is to be understood to mean here a state of the particle filter after conclusion of the actual manufacturing process after which the particle filter is in a serviceable form. A “new condition” is also to be understood to mean a state in which the particle filter is after conclusion of a cleaning process for removing ash deposits accumulated over the course of a relatively long period of use.
The conditioning step is carried out according to the invention for a particle filter that is in such a state before its intended use in its usual operating mode. Upon this intended use, the particle filter, in a state in which it is installed in the motor vehicle and connected to a corresponding combustion engine, filters exhaust of the engine. This has an engine-dependent and/or operating point-dependent particle content of typically 0.1 g to 3 g per km of distance traveled or approximately 1 mg to 100 mg per m3 of exhaust. In this case, the particles typically have an average aerodynamic diameter of 50 nm to 120 nm for a number of approximately 106 to 108 per cm3 of exhaust. In this case, solids particles are to be understood as “particles”. The above-mentioned values with regard to separation efficiencies in the intended conventional use of the particle filter relate to these conditions.
In one embodiment of the invention, the conditioning step comprises passing particle-containing gas through the particle filter such that at least some of the particles contained in the gas are filtered out by the particle filter and are stored in the particle filter. Preferably the particle-containing gas substantially has particles in a size distribution as can also be found upon the intended use of the particle filter for filtering engine exhaust. In this case, provision is preferably made for the particle filter to be exposed to a gas that has a particle content that is increased relative to the value to be encountered in normal operation. The particle filter is thus operated in a particle rich special operating mode when performing the conditioning.
In a further embodiment of the invention, the conditioning step takes place when the particle filter is in the state connected to the combustion engine. The conditioning step is preferably performed before or also directly after the corresponding vehicle has been delivered to the client. In particular, provision may be made to perform the conditioning step before normal driving of the corresponding vehicle. Likewise, it is possible to perform the conditioning step following installation of a particle filter that is cleaned in a separate ash cleaning process into the exhaust system or into the vehicle. In the event that the conditioning step is performed after the vehicle has been delivered, this is preferably carried out by corresponding programming of an engine control unit. The latter is preferably programmed such that the engine operation is modified for a preset initial running distance such that the conditioning is performed.
In particular, when performing the conditioning step in the state connected to the combustion engine, in a further embodiment of the invention, upon performing the conditioning step the combustion engine is operated at least at times such that the exhaust emitted by the combustion engine has an increased particle content compared with normal operation. Preferably, the particle content of the exhaust is increased by about a factor of 1.1 to 10, particularly preferably by a factor of 1.5 to 5, relative to normal operation. In particular, in a further embodiment of the invention, upon performing the conditioning step the combustion engine is operated such that the exhaust emitted by the combustion engine has a particle content of more than 0.01 g per m3 or a number of particles of more than 5×107 per cm3 of exhaust. Thus, the conditioning step can be performed and concluded in a comparatively short time. The setting of an increased particle content in this case can be influenced in the desired manner by the number of and/or the timings of the fuel injections. Additionally or alternatively, the exhaust recirculation rate, charging pressure of a turbocharger, oil or coolant temperature can be influenced such that an increased particle content occurs in the exhaust. The start of injection of a main fuel injection is preferably retarded by about a 10 degree crank angle. The exhaust recirculation rate is preferably increased by more than 1%, particularly preferably by more than 5%. To increase the particle content, a drop in charging pressure by preferably more than 5% relative to the value normally set can be provided. Furthermore, preferably merely for performing the conditioning step, particle forming additives may be added to the fuel and/or the engine oil.
In a further embodiment of the invention, the conditioning step is performed with the particle filter connected to a conditioning device external to the vehicle. The particle filter in such case may be arranged in the exhaust system of the vehicle. The conditioning step can, however, also be carried out on a particle filter arranged outside the vehicle. For performing the conditioning step, the particle filter is connected, for example, to a liquid, gas or solids burner or to a soot generator or a particle generator, which generates a correspondingly desired size range of particles. Thus, when the vehicle is delivered or after installation of a cleaned or exchanged particle filter, a particle filter is available that already has a stably high particle separation efficiency.
In a further embodiment of the invention, particle-containing gas is passed through the particle filter for a pre-settable amount of time upon performing the conditioning step. The performance of the conditioning step is thus limited to a preset amount of time. The preset amount of time is preferably less than one hour, particularly preferably less than 20 minutes.
In a further embodiment of the invention, provision is made that upon performing the conditioning step a particle-containing gas with particles having an aerodynamic diameter of on average less than 0.1 mm is passed through the particle filter. In particular, the particle-containing gas has particles with a substantially smaller diameter than 100 nm. As was able to be demonstrated, with such a particle-containing gas a particularly effective increase in or stabilization of the separation efficiency can be achieved.
In a further embodiment of the invention, after performing the conditioning step, an amount of particles of more than 10 mg per liter of particle filter volume is filtered out from the particle filter and stored in the particle filter. It is preferred that after performing the conditioning step an amount of particles of more than 100 mg per liter of particle filter volume, particularly preferably more than 500 mg per liter, is filtered out by the particle filter and stored in the particle filter. As was able to be demonstrated, a particularly effective increase in or stabilization of the separation efficiency can thereby be achieved.
In a further embodiment of the invention, upon performing the conditioning step a particle-containing gas is used such that the particles stored in the particle filter can be substantially removed during the intended use of the particle filter. Preferably these are particles that then, when the particle filter is being used, can be removed for example by heating to preferably above 250° C. and/or by reaction with constituents contained in the engine exhaust to form volatile products. Solids particles can be used that are capable of volatilizing, for example, by sublimation. Also solids particles can be used that thermally decompose upon the intended use of the particle filter, for example under the action of an elevated temperature, the decomposition products being removable. However, the use of particles which can be removed again from the particle filter can be dispensed with if they are selected such that they do not adversely affect normal operation of the particle filter by their permanent presence in the particle filter. The particles may be formed of solids of one or more of the elements C, H, Al, Si, Ti, Fe, Cu, Pt, Pd, Ce, S, O, N, P, Zn, Ca, Na, Li, Ba, Cl, Rh, F. Initial loading of the particle filter by soot or particles similar to soot is particularly preferred.
Advantageous embodiments of the invention are illustrated in the drawings and will be described below. Therein, the abovementioned features which are still to be discussed below can be used not only in the combination of features indicated in each case, but also in other combinations or on their own, without departing from the scope of the present invention.